Since sinuses are in virtually all land vertebrates, they must have a function. They are covered with a mucosa, epidermal cells which are moist and make mucus. This moistens and warms the air on its way into the lungs. The mucus also picks up particles which come in with the air. A variety of possible functions have been proposed:

Decreasing the relative weight of the front of the skull, and especially the bones of the face.